1. Field of the Invention
This invention relates generally to magnetic random access memory (MRAM) and more particularly to MRAM with multiple layers of magnetic memory cells.
2. Description of the Related Art
MRAM with magnetic tunnel junction (MTJ) memory cells has been proposed for nonvolatile memory, as described in U.S. Pat. No. 5,640,343 and by Reohr et al., “Memories of Tomorrow”, IEEE CIRCUITS & DEVICES MAGAZINE, September 2002, pp. 17–27. In these devices the MTJs are arranged as an array in a single layer (the XY plane) on a semiconductor substrate. In one type of architecture, called a 1T1MTJ MRAM (one transistor and one MTJ), each MTJ is located between a bit line and a transistor, with the word lines located beneath the MTJs. In another type of architecture, called a cross-point (XPC) MRAM, the MTJs are located directly between the bit and word lines. In both MRAM architectures, a selected MTJ cell is “written”, i.e. its magnetic state or +/−X magnetization direction is switched, by write currents passing in X and Y directions through the bit and word lines located above and below the selected MTJ. The write currents generate orthogonal magnetic fields in the X and Y directions that switch the magnetization direction of the selected MTJ.
Stacked or multiple-memory-layer MRAM has also been proposed, but is based on either the 1T1MTJ or XPC architecture for single-layer MRAM. These proposed structures require bit and word lines to be formed above and below each memory layer and may require transistors to be formed in each memory layer. This makes fabrication difficult. In addition, no solutions are known to have been proposed for the problem of cell selectivity in multiple-memory-layer MRAM, i.e., the problem of writing to a selected cell in one memory layer without also writing to non-selected cells in other memory layers in the stack.
Co-pending application Ser. No. 10/735,436, filed Dec. 12, 2003, titled “METHOD AND APPARATUS FOR HIGH DENSITY MAGNETIC RANDOM ACCESS MEMORY (MRAM) WITH STACKABLE ARCHITECURE” and assigned to the same assignee as this application describes a multiple-memory-layer MRAM with electrically conductive access lines located in the same layer as rows of memory cells and on the sides of the memory cell rows. In this structure a single layer of bit lines is located above the stack and a single layer of word lines is located below the stack, with the bit and word lines being connected by columns of memory cells formed by the multiple memory layers in the stack. Writing to a selected memory cell is accomplished by current through the access lines adjacent the selected cell simultaneous with current through the selected cell and thus through the other cells in the column containing the selected cell.
Co-pending application Ser. No. 10/884,914, filed Jul. 6, 2004, titled “MAGNETIC RANDOM ACCESS MEMORY WITH MULTIPLE MEMORY LAYERS AND IMPROVED MEMORY CELL SELECTIVTY” and assigned to the same assignee as this application also describes a multiple-memory-layer MRAM with electrically conductive access lines located in the same layer as rows of memory cells and on the sides of the memory cell rows. In this structure writing to a selected memory cell is accomplished by current through the access lines adjacent the selected cell simultaneous with current through the selected cell, but non-selected cells are prevented from being written by the use of oppositely directed current through the access lines in the memory layer directly above or below the memory layer containing the selected cell.
What is needed is an easily-fabricated, multiple-memory-layer MRAM in which a selected cell can be written without the use of current through the cell and without writing to non-selected cells in the other memory layers.